A thermoelectric heat exchanger assembly is a device based on the thermocouple. An elementary thermocouple comprises an electrical circuit including two dissimilar conductors connected in series. When a direct current (DC) is passed through the circuit, heat is pumped from one of the electrical connections between the dissimilar conductors to the other, giving rise to a hot junction and a cold junction. The converse effect also occurs, i.e. heating one junction and cooling the other will generate a flow of DC in the electric circuit. Reversing the direction of DC flow interchanges the hot and cold junctions.
The effect of an elementary thermocouple is small and a practical thermoelectric heat exchanger assembly requires a large number of elementary thermocouples to be electrically connected in series. This gives rise to an alternating series of hot junctions and cold junctions. A physical structure is thus required to put each cold junction in thermal contact with a cold fluid and likewise to put each hot junction in thermal contact with a hot fluid. These structures are referred to respectively as a hot exchanger and as a cold exchanger. In both cases thermal contact must be achieved by the exchangers without electrically shorting the series connection of elementary thermocouples.
Generally speaking, the dissimilar conductors which give rise to the thermoelectric effect are constituted by suitable alternating P type and N type semiconductor material, e.g. bismuth telluride, and are referred to herein as P type thermoelements and N type thermoelements. P type thermoelements transfer heat in the same direction as the flow of electric current and N type thermoelements transfer heat in the opposite direction. The thermoelements are electrically interconnected by heat conductors which thereby constitute the hot or cold junctions and form parts of the respective exchangers. Although the heat conductors thus form third (and possibly fourth) dissimilar types of conductor in the series connection of thermocouples, their contribution to, or interference with, the thermoelectric effect is minimal.
The above outline recalls the well-known essentials of thermoelectric heat exchanger assembly. The present invention applies more particularly but not exclusively to the type of thermoelectric heat exchanger assembly which keeps the heat conductors at different temperatures, the thermoelements being fed with DC to maintain a temperature difference between the heat conductors. Such a device is called a "heat pump" and can also be used for air conditioning by heating or cooling a fluid by means of heat in the ambient atmosphere.
It has already been proposed to manufacture thermoelectric heat exchanger assemblies which include thermopiles with hot and cold heat exchangers which are electrically conductive and separated alternately by P type and N type thermoelements, the electric current flowing in the pile direction, e.g. vertically. These vertical thermopiles are disposed side by side so as to constitute an assembly with a parallelepipedical appearance in which the small plates or pellets which constitute the thermoelements are disposed in a succession of horizontal planes and the heat exchangers are disposed in intermediate horizontal planes. One of the heat exchanger fluids flows horizontally through each heat exchanger, the cold fluid flowing in one direction and the hot fluid flowing in the other.
The mechanical structure is designed to conciliate the sealing requirements for the fluid flow circuits with differential thermal expansion. It is particularly tricky to produce such a structure since known bismuth telluride based thermoelements are extremely fragile.
Such a structure is described for example in U.S. Pat. No. 3,626,704 (Coe).
Said structure has various disadvantages, in particular when at least one of the fluids, hot or cold, is liquid. A liquid flow circuit can be constituted by a tube which passes through the successive liquid heat exchangers of the thermopile in series, going alternately in one direction and in the other, e.g. in horizontal directions OY and YO, with bent connections situated alternately on one side and on the other of the thermopile. A liquid heat exchanger is a heat exchanger which takes heat from or gives it to a liquid. Two disadvantages then arise. Firstly, if the liquid flow circuit is preformed with a suitably curved tube to impart thereto a zig-zag shape, then the other components of the thermopile, in particular the thermoelements, must be assembled from both sides of the structure which is constituted by said zig-zag tube. This somewhat complicates assembly and increases the cost price. If the liquid flow circuit is not preformed, but rather is constituted by a succession of lengths connected together by glueing or welding after the thermopile has been constituted, there tend to be sealing problems.
A second disadvantage arises when several thermopiles of the above-mentioned type are disposed side by side in a row in the direction OY. Indeed, precautions must then be taken to prevent the bent connections of two neighbouring thermopiles from touching each other, since this would cause electrical short circuits if, as is generally desirable, the successive thermopiles are fed electrically in series. These precautions result either in increasing the distance between the successive thermopiles, i.e. increasing the bulk of the device, or else in complicating assembly, i.e. increasing the cost price.
A further difficulty can arise from the fact that if the tube which constitutes the liquid flow circuit has a circular cross-section of small diameter, the heat exchange area between the water and the tube is too small. If the diameter is increased, this increases the bulk of the thermopile. If the shape of the tube is flattened, its cost price increases and it does not withstand crushing very well when the thermopile is subjected to compression as is desirable to prevent breakage of the thermoelements during handling or during differential thermal expansion.
Further, U.S. Pat. No. 3,196,620 (Ser. No. 343,678, Elfving) describes a thermoelectric heat exchanger whose liquid flow circuit includes straight tubes which pass in series through several thermopiles in the same row. Each stage of each thermopile includes a thermoelement e.g. a P type thermoelement, a (cold) heat exchanger, an (N type) thermoelement and a (hot) heat exchanger. At the end of the row, the straight tube is connected by a bent connection length to another straight tube which passes in series through the thermopiles of another row. It is difficult to assemble such a heat exchanger and the deformation of each thermopile is transmitted to the neighbouring thermopiles. This may cause the thermoelements to break.